Stolen computer

Go to the Apple ID site and click on Visit your Apple ID account page. Sign-in. Under Devices, look for the stolen device name or iMac image, and click on that. It will tell you the serial number so you have something to put on your police report. Apple is not going to track your stolen Mac, and if you did not enable Find My Mac (Find My) on it before it was stolen, then you won't be tracking it either.

If your iMac has been associated with your Apple ID, you might find the serial number by logging in to iCloud, selecting Account Settings > My Devices.

Otherwise, no, there is no other way to retrieve the serial number of the computer.

Is this any help.

https://support.apple.com/en-gb/HT205064